1. Field to the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device for improving the response time of the liquid crystal display.
2. Background of the Invention
Recently, a liquid crystal display (LCD) equipped with thin film transistors (TFT) has developed significantly due to its characteristics including light weight, thin shape and low power consumption. Conventionally, the use of LCDs for PCs was mainly directed to displaying static images, however, along with the progress of such LCDs, they have been substituted for CRTs such as when displaying moving pictures in a graphics system or displaying video images on monitors. Therefore, there is growing interest in displaying moving pictures using LCDs.
While a CRT is in the impulse type of light emission, an LCD is in the hold type with emitting continuous light during a whole period of a frame, thereby being unable to follow the CRT in terms of a quality of moving pictures if leaving them as they are. Accordingly, there have been proposed, for example, a scheme for doubling the refresh rate or the blanking scheme for emitting light intermittently for each frame in order to obtain the similar characteristics to CRT for moving pictures. This is an ideal solution but requires a special liquid crystal with a very fast response, so that the liquid crystals currently in use are not applicable due to their slow response.
For example, a present TN mode TFT-LCD has its on/off response time equivalent to about 1 refresh cycle (16.7 ms at 60 Hz refresh), however, the response time delays greatly in a halftone level, resulting in up to several to ten refreshes. In particular, video images such as TV images mostly have halftone images, so that correct brightness can not be obtained. Even when displaying text data on PCs, it takes a long time for a screen to become a good state where one can easily read if he performs a scroll operation.
As above, a deterioration in image quality when displaying moving pictures on a TFT-LCD results from that a transition of brightness of each pixel does not complete within one frame period of 16.7 ms. Namely, even in the case of liquid crystals with a fast response, the capacitance of the liquid crystal changes in principle of the liquid crystal driving, wherein the targeted brightness can not be reached only by one time of charge/discharge of TFT as long as using the normal driving method, as a matter of course, resulting in the display response being unable to catch up with the image when it changes for each frame. Furthermore, since the response time differs between R, G and B for displaying color images because the response time varies depending on gray levels, a color shift (hue change) may occur in boundary areas of moving edges or thin lines.
There exists a method called overdrive for resolving the delay of the response time. This method is to improve the response characteristics to a step input for the liquid crystal device by applying a voltage greater or smaller than the targeted voltage at the first frame of input changes. For example, Japanese Unexamined Patent Publication No. 1995-20828 discloses a technique that attempts to reproduce faithful brightness with hysteresis and afterimage characteristics being improved even for moving pictures or TV images accompanying active changes by processing input image signals so as to compensate the response characteristics of transmittance against a voltage applied to the liquid crystal, considering a predicted value of voltage response characteristics of the liquid crystal.
The overdrive technique is relatively easily implemented only by changing the driving method and needs no change of a liquid crystal device itself, which is otherwise bothersome. In addition, it is also possible to combine with other techniques for improvement. However, the conventional overdrive techniques including the above publication may simply use a simple voltage value as a parameter. Since the voltage value, when not reaching the equilibrium state, takes the same value for a variety of gray levels of brightness or internal states, it is inappropriate as a parameter for determining a next overdrive voltage.
Furthermore, since the charge is completely discharged in a transition to a full-OFF state, i.e., 0V, there exists no “cumulative response” component, which asymptotically approaches to the targeted gray level as a result of the accumulation of the applied voltage over multiple frames. It is noted that liquid crystals are a viscous fluid and have a slow displacement speed per se, thus this would be regarded as the only cause of slow response time, however, a predicted voltage is 0V insofar as prediction so that it is impossible to represent the process of transition by means of voltage. Though the publication cited above describes about using a low pass filter (LPF) to represent the process of transition in a pseudo manner, a peculiar low pass filter that is different from the one for other gray levels must be provided since the cumulative response does not exist during a full-OFF transition. Furthermore, since the “cumulative response” and “viscosity” are nonlinear over the all gray levels, it is difficult indeed to get the necessary predicted values using the LPF.